This invention relates generally to media handling systems for inkjet printing devices, and more particularly to a media handling system which reduces cockle growth on a media sheet during wet ink printing.
Typically ink-jet printers, or any other printers using wet ink, include a printhead and a media handling system. A print zone, a region where printing occurs, is adjacent to the printhead. The media handling system includes a feed mechanism for feeding a media sheet into and through the print zone. The media handling system also includes a platen which underlies the print zone and supports the media sheet as it passes through the print zone.
During printing, ink is dropped, ejected or otherwise output from the printhead into the print zone and onto a media sheet. Ink used in wet ink-type printing includes a relatively large amount of water. As the wet ink contacts the media sheet, the water in the ink saturates the fibers of the media sheet, causing the fibers to expand, which in turn causes the media sheet to buckle. Such buckling action also is referred to as cockling. Cockling of the media sheet tends to cause the media sheet to bend in an uncontrolled manner downward away from the printhead and upward toward the printhead. Cockling varies the printhead to media sheet spacing (xe2x80x98PMSSxe2x80x99) and the printhead to media sheet angle (xe2x80x98PMSAxe2x80x99). A constant PMSS and PMSA is desired to assure a desired print quality. Varying these measures, as during cockling, can reduce print quality. In the extreme an upwardly buckling media sheet contacts a pen nozzle causing ink to smear on the media sheet. In a worst case scenario an upwardly buckling media sheet in contact with a nozzle damages the nozzle.
Printhead to media sheet spacing (xe2x80x98PMSSxe2x80x99) is defined as the average normal distance from an orifice plate of the printhead to the media sheet (while dry) over the print zone. Alternatively, one or more PMSS""s are defined as the respective normal distances from the orifice plate to the media sheet (while dry) at one or more respective locations within the print zone. Printhead to media sheet angle (xe2x80x98PMSAxe2x80x99) is defined as an average angle between the pen and the dry media sheet, based upon the angle of the orifice plate to a least squares slope of the paper in the print zone. Such angle is measured in a plane normal to the direction of print media carriage travel.
FIG. 1 shows an inkjet pen 10 and a conventional print media handling system 12. The inkjet pen 10 includes a plurality of nozzles 15 for emitting ink at a printhead 14. The print zone 18 occurs adjacent to the printhead in the region of the nozzles. The media handling system 12 includes a drive roller 20, a pinch roller 22, and a platen 24. The drive roller 20 and pinch roller 22 are positioned adjacent to an entrance area of the print zone 18. The pinch roller 22 pushes a print medium 16 to the drive roller 20. As the drive roller 20 rotates, the print medium 16 is driven along the drive roller, then onto and along the adjacent platen 24. The platen 24 includes a contoured region 26 and a generally flat expanse 28. The contoured region 26 is positioned generally adjacent to the printhead 14. The print zone 18 is located between the platen""s contour region 26 and the printhead 14. In operation nozzles 15 drop or eject ink droplets onto an upper surface of the print medium 16 as the print medium is moved in a direction of travel along the platen 24.
Typically the printhead 14 is horizontally positioned so that the nozzles 15 emit ink droplets from an underside of the pen 10. Alternatively, the printhead 14 is angled or vertically oriented with the print medium 16 being correspondingly oriented in the print zone to receive emitted ink droplets. The ink typically includes a large portion of water such that when the ink is printed onto the print medium 16, the ink at times saturates component print medium fibers. This saturation causes the fibers to expand, which in turn causes buckling or cockling of the print medium material.
The media sheet 16 is characterized as having a reverse bow within the contoured region 26 and a droop bow at the edge of the contoured region. The term xe2x80x9creversexe2x80x9d in xe2x80x9creverse bowxe2x80x9d is used because the media sheet curvature is opposite that which is induced by the feed roller 20. The purpose of the varying curvature is to reduce cockling of the media sheet.
According to the invention, a media handling system reduces cockle growth on a media sheet by printing with a reduced number of nozzles while the data rate of incoming data is less than the full or mechanically limited print speed. During a print job, the printer receives incoming data corresponding to characters, symbols or graphics to be printed onto a media sheet. One of the most severe causes of cockle growth on a media sheet occurs when data coming into the printer is delayed during the print job and the printer stops to wait for more data to arrive. During this pause cockle growth continues. Cockle growth moves into the newly wetted areas of the media sheet and may deform adjacent dry areas.
According to one aspect of the invention, the print swath is reduced while the data rate of incoming data is less than the print speed. A reduced print swath is achieved by using only a portion of the nozzles in each row of an inkjet printhead. In particular, the most downstream nozzles are used while the most upstream nozzles are not used. An advantage of only using the most downstream nozzles is that the media sheet area receiving the wet ink is closer to the downstream edge of the print zone and soon away from the printhead. Thus, the wet ink areas where cockle growth is most likely to occur move out of the print zone more quickly (relative to the time ink is first received onto such area). As a result deviations in printhead to media sheet angle (PMSA) and printhead to media sheet spacing (PMSS) are minimized.
The media handling system includes a support along which or over which the media sheet moves in passing through the print zone. An upstream pinch roller is located along the media path prior to the print zone. An optional downstream pinch roller may be located along the media path after the print zone. The support and pinch rollers stabilize the media sheet white the media sheet moves through the print zone. The downstream pinch roller may be of a star wheel configuration to minimize contact with the media sheet and avoid smudging the wet ink on the media sheet. A function of the downstream pinch roller is to hold the media sheet down and away from the inkjet printhead. Another function is to assist in advancing the media, especially once the media sheet trailing edge has passed beyond the upstream pinch roller.
According to another aspect of the invention, a guide shim is operatively positioned with the upstream pinch roller. The guide shim extends along the media path beyond the upstream pinch roller toward the print zone. The guide shim abuts or comes close to the print zone. The location of a lead edge of the guide shim relative to the print zone determines the minimum bottom margin for the inkjet printing device. One function of the guide shim is to provide media advance accuracy as the media sheet trailing edge departs contact with the upstream pinch roller and continues on to the print zone. Another function is to maintain the media flatness as the media sheet continues to the print zone. The guide shim serves to keep the media sheet under the inkjet printhead as the media sheet moves under the printhead. Cockle growth is limited by maintaining such flatness.
According to another aspect of this invention, the guide shim advances with the bottom edge of the media sheet into the print zone. As the guide shim is advanced, it keeps the media sheet in contact with the support, providing advance accuracy, minimal paper to pen spacing, and media trailing edge flatness. The movement of the shim into the print zone also allows the minimum bottom margin to decrease.
According to another aspect of the invention, the support is an endless belt loop driven by drive rollers. Preferably the belt has a ribbing or a grit coating. The media sheet rests on the belt and is stationary relative to the belt while moving through the print zone. The belt provides a continuous surface moving uniformly from the upstream pinch roller to the downstream pinch roller. A belt having ribs serves to reduce cockle growth on the media sheet. A belt having a grit coating, instead of ribs, maintains more accurate referencing between the media sheet and the belt, but is less effective at reducing cockle growth.
According to an alternative aspect of this invention, the support is a stationary platen which extends at least the length of the print zone. The media sheet is fed from the upstream pinch roller onto the platen, through the print zone and to the downstream pinch roller, when present. The upstream pinch roller in combination with a drive roller and the downstream pinch roller in combination with another drive roller advance the media sheet.
An advantage of the method of this invention is that cockle growth is minimized. This is particularly beneficial for media handling system embodiments which do not introduce a reverse bow into the media sheet, such as one which use a flat belt. One advantage of the support, pinch roller, and guide shim configuration is that media advance accuracy is maintained, and cockle growth is controlled, even while the media sheet trail edge leaves contact with the upstream pinch roller. A beneficial effect is that the minimum bottom margin is reduced. An advantage of the shim is that media advance accuracy is maintained even for pinch rollers which do not spin at identical speeds (e.g., due to manufacturing tolerances). These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.